The invention is based on the principle of using a fiber-optical Bragg grating. A Bragg grating is a single mode fibre with a periodical variation in the refraction index over a fibre length of, for example 0.1 to 10 cm. Variation in refraction index is established by illumination of the fibre with an UV-laser. A Bragg grating reflects light by means of a wave length given by the refraction index and the spatial period to the variation of the refraction index (the grating period). Tension in the fibre or temperature will thus give a change of the wave length of the light of the Bragg grating.
In practical use, temperatures in the range -100.degree. C. to +250.degree. C. can be measured at about 20 points along a fiber having a length up to 50-100 km. Using various multiplexing techniques, the number of measuring points can be expanded. Examples of utilization areas are monitoring temperature in power lines, pipelines, transformers, engines, and in industrial processes.
U.S. Pat. No. 5,042,898 describes a temperature compensated Bragg-filter device, where a fiber section having a Bragg grating is mounted between two connected tension means in the filter device. The respective tension devices have different thermal coefficient of expansion and, because of the different temperature dependant expansion of the filter device in the longitudinal direction of the fiber, a relaxation will be established in the tensioned fiber section containing the Bragg grating when there is a rise in temperature. Correspondingly a tension is established at the fiber section with the Bragg grating when then is a temperature decrease. In this way changes are balanced in the wave length as a function of the temperature. An example of use of this known device is wavelength standard.
A number of devices exist for measurement of pressure in fluids. In particular, areas where there is narrow spatial accessibility, high temperature or high pressure, etc. However, the known devices for measuring pressure have disadvantages. For example, to measure pressure in wells, pressure sensors are presently used with a number of electronic components, which in these environments often have low reliability. Besides which, in wells there is often very little space available for installation of additional components, such as pressure sensors. Another example is monitoring of attenuations with electronically based pressure sensors. It has been known for lightning to make the electronics passive and put the pressure monitoring out of order.
Thus, there exists a need for a pressure sensor having mainly passive components and that can be used in stringent environments and confined spaces.